Generally, biological signals with random components or signals embedded in noise such as heart valve signals embedded in extraneous blood flow and muscle signals, or fetal heart beat signals whose shape and amplitude change as the fetus moves in the mother, present special difficulties in measuring. To accomplish such signal measurememt, the signal being measured is often cross-correlated with a known signal similar in shape to the signal being measured.
Utilization of known cross-correlation methods, however, require substantially exact knowledge of the periodic components of the signal to be measured in order to provide a suitable reference signal, and such exact knowledge is difficult to obtain because the frequency, amplitude and shape of such signals often vary over time.